Effects of environmental conditions and standard chlorination practices on the infectivity of Giardia cysts

Abstract

Effects of environmental conditions and standard chlorination practices on Giardia muns cysts were examined using infectivity of cysts to mice as a biological indicator. The eosin exclusion assay consistently indicated high levels of cyst v~ability, despite their total loss of infectivity. Storage of cysts at 4 O C in dry fecal pellets, or soaked in sahne, reduced the infectivity of cysts markedly after 3 d. On the other hand, ~nfectivity of cysts was retained for 20d i f they were isolated from feces by concentration methods, washed and stored in tapwater or wellwater. Routine chlorination procedures for disinfection of drinking water as performed in a modern water treatment plant had no cysticidal effect. This lack of efficacy was not due to a defect in the well-accepted standard procedures followed at this plant, but probably to the high pH of raw water These results raise important questions regarding the epidemiology of giardiasis, because the cysts remained infective for at least 3 wk if free of fecal material. Since most municipalities use only chlorine as a disinfectant, this procedure would favor the spreading of giardiasis if the filtration system is faulty or simply non-existent. Finally, the results obtained in this study confirm those obtained previously and indicate that the eosin exclusion assay is not reliable for determination of giardicidal effects of disinfectants.